Autobot who lost from Cybertron eventually backed down and returned to earth. Humans who travel to the moon accidentally discovered Autobot's spaceship.

Sam after his graduation from college is busy looking for work and left behind by the Bumblebee. Sam inadvertently informed of the arrival of Cybertron to Earth to make humans as slaves.

Optimus Prime is in the earth eventually cooperate with humans to take back the sentinel, the Autobot leader. Sentinel was brought to earth and revived with the matrix belongs to Optimus Prime. Sentinel who eventually defected back to life, and create a portal so that Cybertron can appear on earth.

Is sentinel effort is successful?
This film has graphics and animations are better than ever. But the storyline of the film is flat and unremarkable.
Just watch it! :)